1. Field of the Invention
This invention relates generally to electrosurgical apparatus, and more particularly to an improved polypectome snare for use in excising polyps from an internal body cavity.
2. Discussion of the Prior Art
In U.S. Pat. No. 4,311,143 to Komiya, there is described the construction of a bipolar electrosurgical polypectome snare which comprises an elongated, flexible plastic tube whose outside diameter is sufficiently small to permit it to pass through the lumen of an endoscope and affixed to the distal end of the tube is a first electrode in the form of an annular metallic cap. The second electrode comprises a wire loop which can be extended and retracted relative to the distal end of the tubular body. When inserted through the lumen of an endoscope, it can be made to loop around the polyp to be excised and then by manipulating a hand grip member, the loop can be drawn tight about the neck of the polyp as RF energy is applied between the annular cap and the wire loop. The tissue comprising the polyp completes the circuit between the two electrodes and with sufficient power applied, the stem of the polyp will eventually be cut through.
The device describe in the Komiya Pat. No. 4,311,143 suffers from a number of defects. First of all, because of the disparity in surface area between the small annular cap electrode on the end of the tubular and the larger wire loop, the metallic cap becomes the active electrode and the wire loop merely serves to draw the tissue against the cap. Because of its shape, it tends to desiccate the tissue until the tissue impedance becomes sufficiently high for the wire loop to become the active electrode and effect cutting. This necessarily tends to slow down the cutting action and exposes surrounding healthy tissue to elevated temperatures for a prolonged period.
The Treat U.S. Pat. No. 4,493,320 describes a polypectome snare having a bi-lumen tube dimensioned to fit through an endoscope and where a pair of wires are routed through each of the lumens and extend beyond the distal end thereof where they are joined with an insulating tip member. Thus, portions of the wires themselves become the active bipolar pair.
A problem exists with this construction, however, in that it frequently becomes necessary to rotate the loop at least 90.degree. in those cases where the loop of the snare exits the distal end of the tubular portion in the wrong plane to engage the polyp to be excised. When an effort is made to rotate the plane of the loop by twisting one or the other of the two wires at its distal end, there is a tendency for the loop to twist at its base and thereby create a short circuit condition.